This invention relates to location determination systems. Specifically, the present invention relates to an apparatus and method for coupling data to a position determination device.
Conventional GPS position determination methods and Differential Global Positioning System (DGPS) position determination methods allow for the determination of position with sufficient accuracy for many applications. However, Real-Time Kinematics (RTK) methods are commonly used for applications that require a high degree of accuracy such as, for example, surveying.
RTK systems typically include a base station and one or more rover unit(s). The base station includes a GPS receiver and a radio transmitter that is coupled to the GPS receiver. The base station is located at a known position and communicates with the rover unit via radio. The rover unit includes a radio receiver that is adapted to receive communications from the radio transmitter of the base station and a GPS receiver. The base station periodically sends specially formatted data packets to the rover unit. These specially formatted data packets include GPS data received by the base station GPS receiver.
In a typical surveying application, the surveyor or the surveyor""s assistant carries the rover unit to locations for which position is to be determined. The surveyor, or the surveyor""s assistant then obtains GPS derived position fix data points using the GPS receiver of the rover unit. The rover unit uses, in real time, the data received from the base station in conjunction with GPS data received by the rover unit GPS receiver to determine position relative to the location of the base station GPS receiver within a horizontal accuracy of approximately plus or minus 2 centimeters. The rover unit stores the computed position fix in real-time for later use by the surveyor. In such prior art systems, the wireless data transmission path is one way from the base station to the rover(s).
In a typical surveying application, a surveyor locates the base station at his office, taking the rover unit out into the field to perform the required survey work. If the work area is beyond 50 kilometers from his office, the surveyor must create a new reference point near the work site using well-known survey techniques and the reference point must be chosen so as to assure adequate radio coverage of the site to be surveyed. That is, the reference point must be chosen such that the base station can communicate with rover unit(s) located within the area to be surveyed. It is often very desirable to locate the base station on a high point near or in the work site, in order to get the best radio range coverage possible.
The radios used for communication between the base station and the rover unit(s) are chosen by the surveyor from a small set of radios whose parameters (operating frequency and power) depend on the surveyor""s political status. For example, government agencies have a set of frequency bands available to them that is different from the frequency bands that are available to civilian users. Some frequency bands that are available for civilian use are restricted with regard to transmission power level. This leads to varying coverage ranges, depending on the frequency band used. The ideal coverage range for a surveyor has a minimum of at least five kilometers and a maximum of about 50 kilometers. The maximum is set by RTK performance. That is, at a distance of over 50 kilometers, RTK position determination methods may not provide sufficient accuracy for particular surveying applications.
There are many distinct radio bands available to the surveyor, starting at 30-50 MHz and extending into the VHF range (220 MHz to 450-470 MHz, and 900 MHz). All available frequency bands with the exception of the 900 MHz band requires a license from the Federal Communications Commission (FCC) for permission to operate, and such operation is covered by Part 90 of the FCC rules and regulations. At 900 MHz, there is a special authorization under Part 15, that allows for spread spectrum transmissions with a restricted transmit power of less than one Watt. This low-power level severely limits the range of the radio, which was the FCC""s intention in creating the service. The typical suburban range of a radio operating in the 900 MHz band is about two kilometers, with one kilometer being common.
Surveyors have gravitated towards the 450-470 MHz band because it is possible to obtain a FCC license for nationwide coverage with 35 Watt transmitters. This power level provides the five kilometer minimum coverage range that is needed for effective productivity. The 450-470 MHz band is authorized for primary use by occasional voice service, on a shared use basis with others in a particular service area. Typical users include taxi dispatch, maintenance and repair dispatch, and other business or government services that need occasional voice service and do not need access to a nationwide telecommunications service.
There is a section-of part 90 that permits use of the 450-470 MHz band for telemetry services, subject to some stringent restrictions. These restrictions include the requirement that telemetry users share the channel with other users. However, surveyors typically set up on a work site and start transmitting RTK data packets, which are typically broadcast once per second, with a duration of approximately 0.25-0.4 seconds, until the survey is complete. Thus, a particular channel is virtually occupied by the transient surveyor for a half day, up to as many as five days, depending on the magnitude of the survey job. The problem is severely exacerbated if the surveyor picks a radio channel for which there is a local area repeater system. In this event, if the data packets are picked up by the repeater system they are rebroadcast over a much larger area, thus rendering the particular channel completely useless over a large area. Even if the surveyor""s signal is not repeated, the surveyor""s strong signal may xe2x80x9ccapturexe2x80x9d the repeater""s receiver and thereby prevent voice users from using the repeater.
For the above reasons, a channel formerly used by local users can become unusable for the duration of the survey. When an established user, at his base of operations, cannot access the user""s mobile fleet in a few seconds, his options are a few: wait until the channel clears, or reassign each radio in the fleet to a new channel. This is inconvenient and time consuming for the established user. This also can lead to complaints by the local established users to the FCC for failure to abide by the rules of operation established by the FCC that call for xe2x80x9csharingxe2x80x9d the channel.
In recent years, the number of persons using licensed and unlicensed frequency bands for voice communications has increased dramatically, making these frequency bands crowded, congested and unavailable for use by GPS users. Also, persons using licensed and unlicensed frequency bands for voice communications tend to communicate during business hours and during certain times of the day. At these times, desirable frequency bands become highly congested, particularly in densely populated areas, making usage for surveying activities difficult if not impossible.
The FCC requires that voice users listen before they speak to assure that the channel is available. However, in many instances users do not listen for a sufficient amount of time. This often results in voice users talking over a surveyor""s transmission. This can result in loss of data. If the voice usage is significant, the rover unit may not be able to determine position. In congested areas, this may force the surveyor to change to a less congested frequency band.
What is needed is a method and apparatus that will allow for more efficient use of available frequency bands. More particularly, a GPS system and a method for communicating is required that allows for efficient usage of available frequency bands. Also, a method and apparatus is needed that meets the above needs and that does not interfere with voice communication. In addition, a method and apparatus is needed that meets the above needs and that is inexpensive and easy to operate.
The present invention provides an apparatus and method for coupling position determination system data from a first position to a second position determination system that transmits over unoccupied channels. More particularly, the present invention provides a method and apparatus for monitoring channel usage, automatically selecting an unoccupied channel or sequence of channels for radio transmission of position determining system data and transmitting data over the unoccupied channel.
A position determining network that includes a base station and a rover unit are disclosed. Both the base station and the rover unit include a position determination system (PDS) receiver that is coupled to a PDS antenna for receiving data from a PDS. In one embodiment the PDS that is used is the US Global Positioning System (GPS) that is operated by the US Air Force. Alternatively, other PDS methods and systems can be used such as, for example the GLONASS, Pseudolites, etc.
The base station communicates with the rover unit through a radio that is adapted to receive and transmit over multiple channels. In one embodiment, the radio is adapted to transmit over unlicensed frequency bands. However, alternatively, licensed bands could also be used.
In operation, the base station receives PDS signals that include timing signals. The timing signals are then used by the base station to synchronize timing with the rover unit. This is performed by radio transmission over an established home channel. The base station then monitors multiple radio channels and generates a channel occupancy map that indicates the occupancy of the monitored channels. A channel selection algorithm is then used for determining available channel(s) and assigning one channel, or a sequence of channels for subsequent communications. In one embodiment, user defined channel preference factors are used in the channel selection algorithm for determining assigned channel(s).
A message is then generated that includes data from the received PDS signals and that indicates the assigned radio channel(s). The message is transmitted over the home channel to the rover unit. The rover unit then determines position using PDS signals received at the rover unit and the message received from the base station.
The next transmission is sent and received over the assigned radio channel that was determined to be unoccupied. The base station continues to monitor channel occupancy using subsequently generated channel occupancy maps. The assigned radio channel is updated periodically such that communication is maintained over an unoccupied radio channel. Each time that the assigned radio channel is updated, the updated assigned radio channel is indicated in the message transmitted by the base station.
The rover unit receives each assigned channel and moves to the assigned channel for receiving the next transmission from the base station. Thus, the base station and the rover unit move from channel to channel, communicating over unoccupied channels.
The method and apparatus of the present invention moves from channel to channel, communicating over unoccupied channels. This minimizes channel interference and channel congestion, providing for more efficient use of available frequency bands. By communicating over unoccupied channels, the method and apparatus of the present invention does not interfere with voice communication. In addition, the method and apparatus of the present invention is inexpensive and easy to operate.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments that are illustrated in the various drawing figures.